The advent of video-based home theatre systems has led to an interest in improving the fidelity of the picture obtained from home video media such as video cassettes, video discs, and cable television. At a conference on home video attended by experts from the home video distribution arm of the film industry, a list of defects in the picture fidelity provided by home video was generated. The defects included, in approximate order of severity, reference black level. transient response, low frequency signal-to-noise ratio, luminance level, luminance linearity, hue, saturation, chrominance signal-to-noise ratio, and chrominance/luminance timing.
The process of distributing a motion picture film on home video media involves many production steps between the motion picture film and the home video medium. The defects in home video picture fidelity arise not only because of errors in reproducing the home video medium by the home video equipment, but also because of errors in the home video medium caused by errors in the production process steps between the motion picture film and the home video medium.
It has long been known to use a test signal in the vertical interval of a video signal to determine, and in some cases to correct, errors in the video signal. Such test signals are generally restricted to the distribution environment, but at least one of them, the VIR system, was intended to remain in the video signal distributed to the consumer (exclusively by broadcasters at that time), and to be used by circuitry in the consumer's television for controlling the picture.
The Vertical Interval Reference (VIR) system was introduced in the United States in the early 1970's by GTE as an attempt to correct some of the major chrominance errors that the NTSC system suffered from at the time. As the system was originally proposed, a VIR reference signal was to be added in the vertical interval by every source providing a video signal, and a circuit in the television receiver would adjust the picture in response to the VIR reference signal. The reference signal was to include a caucasian flesh-tone color reference signal on a 70 IRE unit pedestal, a luminance reference at 50 IRE units, and a black reference at 7.5 IRE units. In the version of the system adopted by the broadcasters, the color reference had the same phase as the color reference burst (green-yellow) "for ease of measurement," and the reference signal was added at "a point in the video system where both the correct amplitude and phase of the composite color signal are established and the artistic judgment is made that color reproduction is as desired." Further information on the VIR system, and the above quotations, can be found in A History of the Vertical Interval Color Reference Signal (VIR), EIA TELEVISION SYSTEMS BULLETIN NO. 3, (1975).
Although some broadcasters added the VIR reference signal, and television sets with the VIR circuit were sold, the VIR system as practically implemented did not deliver its intended improvements in picture fidelity. The efficacy of the VIR signal as a reference signal was undermined by the custom of stripping out and replacing the vertical interval at points in the broadcast chain. If the replacement vertical interval included a VIR reference signal at all, the replacement VIR reference signal bore little relationship to the VIR reference signal originally added at the point in the video system where the artistic judgment is made that the color reproduction is as desired. As a result, VIR circuity is no longer available in home video equipment.
Improved automatic picture control systems in consumer video equipment have achieved much of the same reduction in gross chrominance errors that the VIR system was intended to provide. Such automatic picture control systems control the picture by comparing the amplitude and phase of the color burst to an internal reference, and do not use an external picture-related reference.
Home theater systems have recently put new demands on consumer video picture fidelity. Home theatre systems are often viewed with low ambient light levels, as in the cinema. Such viewing conditions enable the video system to reproduce night scenes accurately. However, this also requires that the video system have good luminance linearity at low levels. Luminance linearity errors at low levels compress the grey scale, and make night scenes difficult, if not impossible, to discern.
Under these more critical viewing conditions, the results of chrominance differential gain and phase errors, which cause saturation and hue to change with the luminance level, become more apparent. Such errors are subjectively most noticeable on caucasian flesh tone. A differential phase error of 1 degree is just noticeable in caucasian flesh tones; a differential phase error of .+-.5 degrees represents a flesh tone change from pink to sallow.
Non-linear errors are particularly severe in video disc production, which can typically have a luminance non-linearity of about 10%, differential gain errors in the 12-15% range, and differential phase errors in the .+-.5-6 degree range.
Vertical interval test signals currently available for use in the home video distribution system are designed for convenience of performing measurements, and are insensitive to errors that cause subjectively-noticeable degradation in picture fidelity. For example, the FCC Composite test signal used internally by broadcasters and video production houses includes a luminance bar, 2 T and 20 T pulses, and a 5-step linear luminance staircase with chrominance modulation. This test signal enables errors in parameters such as video gain, low frequency response, sync level, high frequency response, and chrominance gain and delay to be measured and corrected.
The FCC Composite test signal also includes a staircase waveform with five equally-spaced steps for testing luminance linearity. Equally-spaced steps are easy to use for testing luminance linearity since they enable luminance linearity to be measured using an oscilloscope. However, this test signal and other test signals using equally-spaced luminance steps are unsatisfactory because it does not allow the linearity of the system at low luminance levels to be determined accurately.
The staircase waveform of the FCC Composite test signal also includes a chrominance component for determining differential gain and phase errors. This chrominance component has the same phase as the color reference burst, and represents a yellow-green color. This makes differential gain and phase errors easy to measure using a vector scope. However, this method of measuring differential gain and phase is unsatisfactory because the human eye is far less sensitive to differential gain and phase errors in green/yellow colors than it is to differential gain and phase errors in caucasian flesh tones.
As far as is known, none of the presently-used vertical interval test signals is intended to be included in the video signal on home video media for testing purposes in the distribution system, and for error correction purposes in home video equipment.
Systems for distributing motion picture films to the consumer on home video media such as video cassettes, video discs, and cable television are complex. A home video version of a given motion picture film can reach the consumer through the distribution system by one of several different routes. It is often difficult to determine the route by which an individual video cassette or video disc of a given motion picture has reached the consumer. If, for example, the video cassette or video disc is one that has been returned as defective, it is desirable to know the origin of the cassette or disc, so that the source of errors in the distribution system can be determined and remedied.
It is known to add an identification signal to a line in the vertical interval in broadcast television signals. The identification signal simply identifies the source of the signal, for example, the television station from which the signal originated. All signals originating at that source include the same identification signal. It would not be satisfactory to adopt this identification system in a video distribution system since the identification system would not enable the distribution process to be identified with sufficient accuracy.